A communications system may be based on one or more communication technologies, such as e.g. Global System for Mobil Communications (GSM), Long Term Evolution (LTE), General Packet Radio Service (GPRS), Enhanced GPRS/Enhanced Data rates for GSM Evolution (EGPRS/EDGE) etc. EGPRS/EDGE is an enhancement of a GSM network in which EDGE is introduced on top of the GPRS that is used to transfer data in a packet switched mode on several time slots. EGPRS2 is a phase 2 of the EGPRS, i.e. a further development of EPGRS.
The enhancement of GSM and EDGE in order to provide support for MIMO has been proposed in several occasions. A two layer transmission is already supported in GSM, in a feature called Voice services over Adaptive Multi-user channels on One Slot (VAMOS). The simulation results of the enhancements with VAMOS are intriguing and suggest that MIMO for EGPRS/EGRPS2 should be given careful consideration. As known for a skilled person is the use of multiple antennas at both the transmitter and receiver in order to improve communication performance.
In the context of MIMO (either single user MIMO or multi-user MIMO) a layer refers to a data stream (this is the MI part in MIMO). These data streams are the Multiple Input. This means that the term “two or more layers” refers to “two or more data streams” and these data streams are different.
One enhancement presents a straightforward implementation of MIMO for EGPRS that is to a large extent backwards compatible with the GSM and EDGE air interface. Each layer is independently coded and modulated according to an EGPRS modulation and coding scheme. Each layer is assigned a different training sequence. The standardized VAMOS training sequence pairs are proposed (after a straightforward mapping of the training bit sequence to antipodal 8 Phase Shift Keying (PSK) symbols). Thus, the transmitter comprises two parallel EGPRS transmitters, each fed its own data stream and training sequence. PSK is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave).
Beamforming is a signal processing technique where radio signals transmitted over an air-interface are combined by multiple transmit antennas in such a way that the electromagnetic waves transmitted or received at some angles experience constructive interference while the electromagnetic waves transmitted or received at other angles experience destructive interference. The technique may be used at both the transmitting and receiving ends in order to achieve spatial selectivity. More generally, modern beamforming techniques are based on the principle that when both the transmitter and receiver have knowledge of the propagation channel, the weight for each transmit antenna may be adapted so that the Signal to Noise Ratio (SNR) at the receiver is improved. When the weights affect only the polarization of the transmitted radio wave, beamforming cannot be interpreted as forming a physical beam and may not yield spatial selectivity.
In spatial multiplexing, multiple data streams are transmitted from different transmit antennas but in the same frequency channel. If the propagation channel has a sufficient number of degrees of freedom (i.e. the rank of the channel matrix is large enough), the receiver may separate these streams into (almost) orthogonal channels.
Codebook Based Precoding
The codebook based precoding is a technology used in e.g. LTE which fixes a common codebook comprising a set of vectors and matrices known apriori to both the transmitter and the receiver. The vectors and matrices may also be referred to as codewords. In other words, a finite set of precoding matrices is called a codebook. Each precoding matrix in the codebook is associated with an index. In a closed-loop codebook based precoding transmission, the receiver transmits, to a transmitter, the index of a precoding matrix from a pre-designed codebook.
Closed Loop MIMO in LTE
Closed loop MIMO technologies have been standardized in LTE. LTE requires the calculation of three feedback quantities at the receiver, namely, Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI) and Rank Indicator (RI), in order to perform channel adaptation at the transmitter. The CQI is used to select a modulation and coding scheme. The PMI is used to select the codebook index. The RI indicates the preferred number of layers. Since the coherence time of the radio channel is in the order of a few ms, LTE has been designed to support fast feedback. One aim of closed loop spatial multiplexing transmission modes in LTE is to adapt the transmission to the current (instantaneous) channel conditions. Channel state information just a few sub-frames old (1 sub-frame has duration of 1 ms) may be already obsolete. The periodicity of the feedback loop is in the order of a few ms.
Reconfigurable Multiple Antennas in the Receiver
Recent measurement campaigns on MIMO channels have revealed the potential benefits in capacity performance that may be obtained by adapting the antenna configuration at the receiver. The results show that choosing the best among three possible antenna configurations at the receiver along 20 m route sections, leads to significant gains. The speed of the receiver did not exceed 30 km/h, and the frequency band was 2.65 GHz. This means that the best antenna configuration was kept fixed during time intervals of 2.4 s or longer. It is reasonable to expect that when the measurements had been performed in the 900 MHz band, the same gains would have been obtained by keeping the antenna configuration fixed during time intervals longer than 2.4 s, perhaps up to 7 s.
Spectrum and power efficiencies are of paramount importance in wireless communications. Therefore, it is desirable to implement closed loop MIMO techniques for MIMO in EGPRS/EGPRS2. Moreover, when MIMO for EGPRS/EGPRS2 is to be standardized and deployed, it is important to ensure that it is designed to maximize the link performance, while maintaining, to a large extent, backwards compatibility with the GSM air interface.
Closed loop techniques such as those used in LTE and other wireless technologies are very promising, but cannot be applied to an enhancement of EGPRS/EGPRS2 with MIMO, because the GSM air interface does not support the low latency required by the fast feedback channels. In other words, the gains brought about by the closed loop MIMO techniques standardized in LTE cannot be achieved in a MIMO EGPRS/EGPRS2 system. Moreover, in LTE the transmission antennas in the base station broadcast cell specific reference signals which are essential for the receiver in order to compute the CQI, PMI and RI. Such signals are not available in the current GSM/EDGE air interface.